1. Field of the Invention
This invention relates to an electromagnetically driven shutter in which the exposure operation and returning operation of the shutter leading member and the shutter trailing member are performed by electromagnetic force.
2. Description of the Prior Art
Greater utilization of electronics in the camera art has made it necessary to simplify electromagnetically driven shutters of this type. Various proposals have been made for this purpose.
For example, slit exposure shutters whose leading and trailing shutter curtains form a slit opening very often actuate the leading shutter curtain and trailing shutter curtain independently of each other. To achieve this end, two separate electromagnetic devices are often used. However, it is important that the camera using the shutter be capable of employing a compact, low voltage battery of limited capacity. Therefore, the total amount of electric energy available to a shutter from a battery is restricted. In order to prolong the life of the battery, it is necessary to increase the efficiency of the shutter's electromagnetic drive source itself. Since the spare space in the interior camera of a housing is comparatively small, it is difficult to employ a large, high efficiency shutter drive. Thus, one would wish to use the highest voltage possible to obtain the strongest driving force from a smaller electromagnetic device. A great many slit exposure shutters need driving forces greater than that of ordinary lens shutters. This increases the demand made by the electromagnetic device upon the current supply.
The electric current flowing through the moving coil of the electromagnetic device must be reversed in direction as the exposure and resetting operations of the shutter alternate each other. A typical example of a conventional arrangement is shown in FIG. 1. That is, L is a coil of the electromagnetic drive device for the shutter leading member only (another coil for the shutter trailing member is not shown here). Tr1, Tr2, Tr3 and Tr4 are driving transistors of the same polarity (NPN) connected to each other to form a bridge. Ca and Cb are control signal sources for the transistors Tr1 to Tr4. When either one of terminals, a and b, is fed with an actuating signal, the corresponding control signal source produces an output or control signal of low level. +Vcc and -Vcc are positive and negative buses connected to the positive and negative terminals of a battery (not shown) respectively. With the circuit of FIG. 1, when the actuating signal is applied to the terminal, a, but not to the terminal, b, the transistors Tr1 and Tr4 are turned on, while the other transistors Tr2 and Tr3 remain off, thereby the coil L is supplied with current flowing in a direction indicated by arrow. Conversely when the terminal b is fed with the actuating signal, the transistors Tr3 and Tr2 are turned on, and the Tr1 and Tr4 are turned off, thus reversing the direction of current flow, and driving the coil L to move in the opposite direction to that when the preceding operation is carried out.
Since, in the illustrated circuit, connected across the both terminals +Vcc and -Vcc of the battery is a series circuit of the output impedances of the two transistors and the impedance of the coil L, the voltage of the battery less 2 times the voltage drop of the output circuit of the transistors is applied across the both ends of the coil L, and, therefore, the driving force exerted by the coil L is lowered by a magnitude proportional to 2 Vo. As the driving transistors constituting the drive circuit of the camera it is difficult to select ones of too high output power for employment. Therefore, since the impedances of the transistors Tr1 to Tr4 of this circuit cannot be ignored in respect to the impedance of the coil L, in order to cause a sufficient drive current to flow through the coil L, the battery voltage must be higher by the voltage drop due to these transistors.
We see again that the conventional transistorized reversible drive circuit suffers from the voltage drop due to the transistors and when supplied with current from a camera battery or like electrical power source of small capacity with a low voltage cannot produce a sufficiently large drive current, thereby giving a disadvantage that the shutter operates with the lack of stability and at a slower speed than is desired.